That's a rather narrow perspective. I believe the actual narrative was that her killing Trask at a specific event triggered a dismal future for mutantkind. There's nothing to say killing him another time would have had the same effect, not that I would ever justify killing someone. And you'd have to be completely ignorant to think someone like Trask would care if she were dead or alive if he got what he wanted.
But you do have a point. And I'm sorry for the situation you were in. Yet it would have been better to work as a team and find the right solution together.